widely used method. The rise of this technology goes back to 2007 when a
description of the process for creating hydrophobic pattern in paper and using
hydrophilic channels for pumpless liquid transport was reported by Whitesides
and co-workers [ 89 ]. Hydrophobic barriers can be made using wax, SU-8 and
other photoresist materials, PDMS, alkenyl ketene dimer [ 5 ], polystyrene, ethyl
cellulose, silicones, rosin, paraffin, printer varnish, cellulose esters, hydrophobic
gels [ 90 ] and possibly others. All these materials are impervious to water and allow
for implementation of various structures for transport and storage of liquids. Width
of channel structures that can be achieved by these methods varies and can go down
to 200-300μm with some optimization.
6.1.1 Wax Printing
Wax printing [ 91 , 92 ] is the most commonly used technique. It is easy, fast, and low
cost, and can be easily applied for small-scale manufacturing. Typically, the
channel width is in the range of 1–5 mm. This technology is very straightforward:
a design is sent to a printer and printed directly on a sheet of paper, this paper can be
sandwiched between a metal foil and placed on a hot-plate at ca. 70C for 1 min.
For better penetration of wax into paper it can be baked from both sides. Baking
temperature, baking time and printing mode are parameters that can be used for
optimization in each specific case. Speading of wax in paper after bake can be
Fig. 7.6 Examples of paper-based structures fabricated by (a) wax-printing (Adapted from [ 92 ]
with permission of Royal Society of Chemistry), (b) screen-printing (Adapted from [ 99 ] with
permission of Royal Society of Chemistry), (c) inkjet-printing (Adapted from [ 104 ] with permis-
sion of John Wiley and Sons) and (d) photolithography assisted method (FLASH) (Reproduced
from [ 108 ] with permission of Royal Society of Chemistry)
7 Paper Microfluidics 179